In the catalytic oxidation of ethylene to ethylene oxide, the start-up operation of a highly selective Ag-based catalyst requires a special procedure. Specifically, the catalyst, especially when rhenium is used as a promoter, requires an initiation period before it is able to give the expected higher performance.
U.S. Pat. No. 4,874,879 to Lauritzen et al. and U.S. Pat. No. 5,155,242 to Shanker et al. disclose start-up processes in which freshly prepared rhenium (Re)-containing Ag-based catalysts are chlorinated before adding oxygen to the feed. That is, the '879 and the '242 patents disclose a start-up process in which the catalysts are pre-chlorinated prior to the introduction of oxygen into the feed. In particular, the catalysts were, initially, pre-chlorided with a feed containing ethylene, methane and ethyl chloride. This was followed by the addition of oxygen to the feed, at which time the reaction temperature was kept below 273° C. Eventually several adjustments to the reaction conditions were made in order to obtain an optimum performance. This prior art pre-chloriding step reportedly provided enhanced activity of the Re-containing catalysts and allowed the ethylene oxide reaction to start-up at low temperatures.
A further start-up process is disclosed in U.S. Patent Application Publication No. 2004/0049061 A1 to Lockemeyer et al. In particular, this disclosure provides a method for improving the selectivity of a supported highly selective epoxidation catalyst comprising Ag in a quantity of at most 0.17 g per m2 surface area of the support. The improvement was achieved by contacting the catalyst with a feed including oxygen at a catalyst temperature above 250° C. for duration of up to, at most, 150 hours.
A yet further start-up process is disclosed in U.S. Pat. No. 7,102,022 to Lockemeyer et al. Specifically, the '022 patent discloses a method for the start-up of a process for the epoxidation of an olefin comprising an Ag-based highly selectivity epoxidation catalyst. The method disclosed in the '022 patent includes contacting a catalyst bed with a feed comprising oxygen. In this treatment, the temperature of the catalyst bed was above 260° C. for a period of time of, at most, 150 hours.
Despite the above start-up procedures, and because of the importance for operating Ag-based highly selective catalysts under optimum performance conditions, there is a continued need to develop new and improved methods that can be used for the start-up of a process for the epoxidation of olefins, especially ethylene.